Water cryptate

The remarkable protonation features of 21 led to the formulation of the diproto-nated species as the water cryptate, [H20 cr 21, 2H+] 25, in which the water molecule accepts two +N-H---0 bonds from the protonated nitrogens and donates two 0-H---N bonds to the unprotonated ones [2.17, 2.96], The second protonation of 21 is facilitated by the substrate it represents a positive cooperativity effect, mediated by H20, in which the first proton and the effector molecule water set the stage both structurally and energetically for the fixation of a second proton. When 21 is tetraprotonated it forms the chloride cryptate cryptate [Cl c 21,4H+] 26, in which the included anion is bound by four +N-H---X- hydrogen bonds [2.97] (see also Chapt. 3). 

A study of the protonation features of (5) indicated that the second protonation was easier than the first one and that proton exchange was very slow. The results agree with the formulation of the diprotonated species as a water cryptate [H2O c(5)-2H ] (11), in which the water molecule is held in a tetrahedral array of hydrogen bonds, accepting two bonds from the protonated nitrogen sites... 

For non-aqueous solvents, the formation rates for the alkali metal cryptates are not greatly solvent-dependent (Cox, Garcia-Rosas Schneider, 1981). However, a comparison of the rates for methanol with those for water indicates that the latter are considerably slower (Cox, van Truong Schneider, 1984) and are, indeed, much slower than expected... 

In water, the relatively low stability of the alkali metal and alkaline earth cryptates (except those for which there is a near-optimal fit of the cation in the intramolecular cavity) has resulted in difficulties in undertaking a wide-ranging kinetic study in this solvent. However, in non-aqueous media, the stability constants are larger and most of the studies have been performed in such media. 

The kinetics of formation and dissociation of the Ca2+, Sr2+ and Ba2+ complexes of the mono- and di-benzo-substituted forms of 2.2.2, namely (214) and (285), have been studied in water (Bemtgen et al., 1984). The introduction of the benzene rings causes a progressive drop in the formation rates the dissociation rate for the Ca2+ complex remains almost constant while those for the Sr2+ and Ba2+ complexes increase. All complexes undergo first-order, proton-catalyzed dissociation with 0bs — kd + /ch[H+]. The relative degree of acid catalysis increases in the order Ba2+ < Sr2+ < Ca2+ for a given ligand. The ability of the cryptate to achieve a conformation which is accessible to proton attack appears to be inversely proportional to the size of the complexed metal cation in these cases. 

The study of Lehn s cryptands has shown that a three-dimensional arrangement of binding sites leads to very stable inclusion complexes (cryptates) with many cations. For example, the stability constant for K+ in methanol/water (95/5) is five orders of magnitude higher with [2.2.2]-cryptand [37] (log K 9.75 Lehn and Sauvage, 1975) than with [2.2]-cryptand [38] (log... 

In the cryptate [Eu(2.2.2)(H20)2] the encapsulated metal ion is ten-coordinate and has two inner shell water molecules (165,182). The exchange rate constant is the lowest of all Eu " " chelates measured so far which is probably due to the positive charge. The value of AV is close to zero and therefore an I mechanism was assigned for the water exchange process (Table X). 

Finally, a macrobicyclic or [2]-cryptate effect is found by comparing the stability of the K+ complex of 30 with that of 22, where the solvation shell is completed by solvent molecules (a better model would be a ligand of type 22 bearing a — CH2CH2OCH2CH2OCH3 chain on one nitrogen) a stability increase of more than 105 (in methanol/water, 95/5) is found on introduction of the third bridge. 

Comparing ligands 22 and 38 which contain the same number of binding sites, we note that the special complexation features of 38 result from the cryptate nature of its complexes. Indeed, whereas in complexes of 22 polar solvent molecules may approach the cation from top and bottom, it is much more shielded in complexes of 38. This difference in behaviour is reflected in the corresponding change in ligand thickness (Table 12). The results in Table 11 also display the expected decrease in M2+/M+ stability ratio as the dielectric constant decreases from water to methanol. 

Polymer phase-transfer catalysts (also referred to as triphase catalysts) are useful in bringing about reaction between a water-soluble reactant and a water-insoluble reactant [Akelah and Sherrington, 1983 Ford and Tomoi, 1984 Regen, 1979 Tomoi and Ford, 1988], Polymer phase transfer catalysts (usually insoluble) act as the meeting place for two immiscible reactants. For example, the reaction between sodium cyanide (aqueous phase) and 1-bromooctane (organic phase) proceeds at an accelerated rate in the presence of polymeric quaternary ammonium salts such as XXXIX [Regen, 1975, 1976]. Besides the ammonium salts, polymeric phosphonium salts, crown ethers and cryptates, polyethylene oxide), and quaternized polyethylenimine have been studied as phase-transfer catalysts [Hirao et al., 1978 Ishiwatari et al., 1980 Molinari et al., 1977 Tundo, 1978]. 

Cryptands, 42 122-124, 46 175 nomenclature, 27 2-3 topological requirements, 27 3-4 Cryptate, see also Macrobicyclic cryptate 12.2.2], 27 7-10 applications of, 27 19-22 cylindrical dinuclear, 27 18-19 kinetics of formation in water, 27 14, 15 nomenclature, 27 2-3 spherical, 27 18 stability constants, 27 16, 17 Crystal faces, effect, ionic crystals, in water, 39 416... 

In the complexation reaction cryptand must compete with solvent molecules for the cations in solution. Thus solvents such as methanol with low dielectric constant and solvating power offer a preferrable reaction environment but we have achieved quantitative yields in water. The main problem encountered in syntheses of cryptates has been the presence of other cations such as Na and KT competing for the cryptand. Care is taken to minimize the concentration of competing cations of size similar to the cation intended for complexation by using lithium salts for buffering solutions.-... 

We envision several potential generator-produced radionuclide labels for cryptates (Table I). Fortunately, early evaluations can be performed more conveniently with longer-lived tracers that are commercially available. The cryptate complexes are conveniently formed from the metal in deionized water and the cryptand dissolved in water or methanol. The complexes form instantly upon... 

Table 8 Log Stability Constants of Some Cryptate Complexes in Water, Log Kl...

Figure 26 Stability constants (log Ka) of the alkali cryptates (left, in 95 5 methanol/water, M/W, or in pure methanol, M, at 25 °C) and of the alkaline earth cryptates (right, in water at 25 °C) (reproduced with permission from...

These complexes, unlike the crown ether complexes but similar to the aza-crown and phthalocyanine complexes, are fairly stable in water. Their dissociation kinetics have been studied and not surprisingly they showed marked acid catalysis.504 Association constant values for lanthanide cryptates have been determined.505,506 A study in dimethyl sulfoxide solution by visible spectroscopy using murexide as a lanthanide indicator showed that there was little lanthanide specificity (but surprisingly the K values for Yb are higher than those of the other lanthanides). The values are set out in Table 9.507... 

The stability and selectivity patterns of cryptands were found to be markedly solvent dependent and stability constants of Ag[2]cryptates in a range of solvents are presented in Table 62.474"478 Thermodynamic data for their formation in water are given in Table 63.476... 

Table 63 Thermodynamic Data for the Formation of Silver [2]Cryptates in Water at 25 °C476...

Figure 12. Crystal structure of the bromide complex with 6 showing the side (A) and end-on (B) views for the bromide and one water molecule, and the side (C) and end-on (D) views for the bromide and three water molecules in the two crystallographically independent cryptates.

In the analogous tetrahedral dinegative oxoanion cryptates a similar mix of direct and indirect water-bridged H-bonds links the encapsulated oxoanion with the NH1 donors. The direct NH+ 0" contacts are (Table 1) on average, slightly shorter than those... 

Finally the structure of the tosylate ciyptate of R3Bp demonstrates the size exclusion expected of the large cation (Fig. 11). Even so, it is not devoid of interaction with the cationic host, in the lattice at least, as each anion exhibits one moderately short H-bond contact to one of the NH1 functions of the cryptate. These direct H-bond contacts are often supported by indirect water-mediated links of shorter dimensions, acting as part of branched hydrate chains which run through the less hydrophobic section of the lattice. 

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